1. Field of the Invention
The invention relates to metal oxides which have a high positive surface charge and the preparation and use thereof.
2. Description of the Related Art
The surface charge properties of particulate metal oxides are important properties when these substances are used for controlling the flow properties and triboelectric charge in toners, developers and powder coats.
Particulate metal oxides having electrostatically positive chargeability are used in particular as constituents of developers and toners for visualization of negative electrostatic charge images. A precondition is high, stable, uniform positive triboelectric chargeability of the particles.
Particulate metal oxides having electrostatically positive chargeability are used in particular as constituents of powder coats for controlling the charging in the triboelectric application of powder coats. A precondition is again, high, stable, uniform positive triboelectric chargeability of the particles.
Particulate metal oxides having electrostatically negative chargeability are used in particular as constituents of developers and toners for visualization of positive electrostatic charge images. A precondition is high, stable, uniform negative triboelectric chargeability of the particles.
Particulate metal oxides having electrostatically negative chargeability are used in particular as constituents of powder coats for controlling the powder flow properties in the application by means of the corona process.
The sign, the magnitude and the density of the surface charge of particulate metal oxides are decisively determined by the chemical structure of the particle surface. In the case of metal oxides, such as silicon dioxides, aluminum oxides or titanium dioxides, surface hydroxyl groups are the charge-determining groups, the acid strength of the metal-hydroxyl groups decisively influencing the sign of the charge and the surface charge density. Thus, unmodified silicas or metal oxides modified with alkylsilyloxy groups are particles which can be triboelectrically charged to have a negative charge.
For producing positive triboelectric charges, as described, for example, in DE 33 30 380, aminosilanes can be chemically fixed on the particle surface. However, the positive triboelectric charge build-ups achieved with particles modified in this manner tend to have a low magnitude and are greatly dependent on the ambient conditions, such as the atmospheric humidity.
More stable triboelectric charging can be achieved by additional hydrophobization of the particles, as described, for example, in DE 33 30 380 or DE 37 07 226. However, in these cases too, the magnitude of the positive triboelectric charge build-up is frequently small.